1. Field of the Invention
The present invention relates to a method for manufacturing compound semiconductor device. Particularly the invention relates to a method for manufacturing semiconductor device using especially AlInAs compound, which prevents characteristic deterioration caused by invasion of F (fluorine) especially to compound semiconductor.
2. Description of the Related Art
AlInAs compound semiconductor is expected as semiconductor material for ultra high-speed device of next generation such as HEMT (high electron mobility transistor) and HBT (Hetero-junction Bipolar Transistor) because of high grating coordination to InP substrate and physical characteristic such as low trap density. However a problem is known that F (fluorine) taken out from environment atmosphere including air diffuses inside at heat treatment of manufacturing process and connects to donor such as Si so as to inactivate it.
That is, in HEMT, after loading a gate electrode, AlInAs layer exposes at periphery thereof and is exposed to air. In HBT, just before forming an emitter electrode, n-AlInAs sometime exposes. Further in case of separation between elements by mesa structure, it can not avoid that a section of AlInAs layer exposes at a sided face of mesa.
For such problem, a method is proposed that so called barrier layer by AlAs/InAs, In(AlGa)As, GaInAs, etc. is inserted into AlInAs or grown on upper layer. However as grating coordination is not formed in such method, barrier layer thicker than critical coat thickness is not formed so as not to depress enough invasion of F finally. Further there is not any effect against F invasion to the side face of mesa in case of mesa structure.
Although F removing by sealing by SiN coat or ammonium sulfide process is proposed else, actually the effect thereof is not obtained enough.
In order to solve the above-mentioned prior art, an object of the invention is to provide a method for manufacturing compound semiconductor device, which depresses effectively F invasion to compound semiconductor material including AlAs or InAs.
According to the invention, a method for manufacturing compound semiconductor device comprises a process carrying out plasma treatment exposing surface of compound semiconductor material including AlAs or InAs exposing in atmosphere at manufacturing process in plasma of gas including any of P (phosphorus), N (nitrogen), H (hydrogen), and Ar at substrate temperature of less than 200xc2x0 C.
The method concerning the invention has a main characteristic in a point to carry out plasma treatment at low temperature less than 200xc2x0 C. to exposed compound semiconductor material. By desirable mode of a method according to the invention, a second plasma treatment carried out continuously after the above-mentioned plasma treatment at higher temperature without exposing compound semiconductor material in air.
That is, there is a case frequently that F taken from air has already attached in exposed compound semiconductor material at actual manufacturing process of compound semiconductor. Therefore when substrate temperature is raised, the F invades into compound semiconductor material even for forming a protecting coat. Then it is important to remove F attaching at surface of the compound semiconductor with plasma treatment by gas including P or inactive gas of N2, H2, Ar, etc. at low temperature less than 200xc2x0 C. which prevents invasion of F first. Here limitation of less than 200xc2x0 C. of substrate temperature at plasma treatment is determined by that activation energy of F (fluorine) invasion is 1.0 eV. A carrier density change of n-AlInAs material due to the thermal annealing is defined by the following equitation (A paper titled xe2x80x9cDegradation mechanism of the AlInAs/GaInAs high electron mobility transistor due to fluorine incorporationxe2x80x9d written by N. Hayafuji et. al., published by Americal Institute of Physics, 1996, pages 4075-4077).:
n/n0=exp(xe2x88x929180xc2x7txc2x7exp(xe2x88x921.0/kbxc2x7T))
where, n: the concentration of free electron (cmxe2x88x923) in n-AlInAs material; n0: the initial concentration of free electron (cmxe2x88x923); kb: 8.617385xc2x710exe2x88x925[eV/K] (Boltzmann constant).
If the substrate temperature is 200xc2x0 C. and the treatment time is one hour, i.e., T: 473 [K] and t: 3600 [sec], the carrier density change is calculated at 0.999 on the basis of the above-mentioned equation. That is, plasma treatment of less than 200xc2x0 C. and within one hour depresses F invasion such that carrier density of AlInAs is changed, in other words, the amount of F invasion is 10% of the impurity amount of n-AlInAs or less.
Reason why surface-protecting coat by SiN proposed in related art does not obtain enough effect is assumed that F having attached before forming the surface-protecting coat has invaded into compound semiconductor material because substrate temperature is high at forming SiN coat.
After removing F of surface of compound semiconductor material as above-mentioned, stable compound is formed at surface of compound semiconductor material so as to stop new attachment or invasion of F when a second plasma treatment is carried out at normal high substrate temperature with gas including P, for example, phosphine (PH3) gas. It is desirable that the second plasma treatment is continuously carried out without exposing the compound semiconductor material from plasma treatment carried out at said low temperature so that cleaned surface of compound semiconductor material is not invaded again by F.
The above-mentioned plasma treatment is suitably carried out repeatedly at manufacturing process of compound semiconductor device. That is, in HEMT, n-AlInAs layer exposed at channel domain just before forming a gate electrode becomes an object of plasma treatment. In HBT, besides the plasma treatment is carried out about n-AlInAs layer before loading emitter electrode, plasma treatment is useful when side wall of mesa exposes showing head of the base.
Removing of F by ammonium sulfide treatment proposed in related art is assumed not to have enough effect because it is not avoided that compound semiconductor material is exposed again in air after the treatment.